Combustion devices

ABSTRACT

A FLAME STABILIZER COMPRISES AN ANGLE SECTION FIXED WITH ITS APEX POINTING UPSTREAM. AT EACH OF ITS DOWNSTREAM EXTREMITIES IS A BIMETALLIC PLATE, THE TWO PLATES EXTENDING DOWNSTREAM IN THE DIRECTION OF FLOW AND PARALLEL TO EACH OTHER. A PILOT BURNER IN THE FORM OF A FUEL SPRAY BAR EXTENDS TRANSVERSELY OF THE STABILIZER WITHIN THE ANGLE SECTION IN WHICH THERE ARE SMALL HOLES TO ADMIT AIR TO THE BURNER. THE INNER FACES OF THE BIMETALLIC PLATES ARE OF NICKEL ALLOY HAVING A HIGH COEFFICIENT OF EXPANSION AND THE OUTER FACES ARE OF A REFRACTORY METAL SUCH AS NIOBIUM OR MOLYBDENUM HAVING A LOW COEFFICIENT OF EXPANSIONS. THE DIFFERENTIAL EXPANSION OF THE PLATES   UNDER THE EFFECT OF HEAT CAUSES THESE TO BEND OUTWARDLY TO INCREASE THE FRONTAL AREA OF THE STABILIZER AND CAUSE INCREASED TURBULENCE.

United States Patent [72] Inventor William Dean Bryce Farnham, England[21] Appl. No. 823,442 [22] Filed May 9, 1969 [45] Patented June 28,1971 [32] Priority May 16, 1968 [33] Great Britain [31] 23298/68 [73]Assignee Minister of Technology in Her Britannic ajestys Government ofthe United Kingdom of Great Britain and Northern Ireland, London,England [54] COMBUSTION DEVICES 10 Claims, 1 Drawing Fig.

[52] US. Cl 60/39.72, 29/1955, 60/261, 431/350 [51] Int. Cl E02k 3/10[50] Field of Search 60/39.72, 261; 29/194, 195.5, 198(1nquired);43l/350 [56] References Cited UNITED STATES PATENTS 2,332,41610/1943 Waltenburg 29/1955 2,563,270 8/1951 Price 60/261 2,578,19712/1951 Mudge 29/195.5 2,679,137 5/1954 Probert 60/3972 2,770,09611/1956 Fox 60/3972 2,805,192 9/1957 Brenner.... 29/198 2,918,79412/1959 Hurd 60/3972 2,974,488 3/1961 Eggers... 60/3972 3,057,04810/1962 Hirakis 29/198 Primary Examiner-Douglas Hart Att0rneyStevens,Davis, Miller and Mosher ABSTRACT: A flame stabilizer comprises an anglesection fixed with its apex pointing upstream. At each of its downstreamextremities is a bimetallic plate, the two plates extending downstreamin the direction of flow and parallel to each other. A pilot burner inthe form of a fuel spray bar extends transversely of the stabilizerwithin the angle section in which there are small holes to admit air tothe burner. The

inner faces of the bimetallic plates are of nickel alloy having a highcoefficient of expansion and the outer faces are of a refractory metalsuch as niobium or molybdenum having a low coefiicient of expansion. Thedifferential expansion of the plates under the effect of heat causesthese to bend outwardly to increase the frontal area of the stabilizerand cause increased turbulence.

COMBUSTION DEVllCES The present invention relates to combustion deviceswherein a flame stabilization zone is formed in a swiftly moving gasstream by the separation of flow from the surface of a body disposedwithin the stream.

In aircraft gas turbine jet propulsion engines, extra power can bedeveloped by burning additional fuel in an afterburner. It has long beenthe practice in such installations to use fixed baffles to producestable turbulent (or recirculation) zones in their wakes wherein flameswill maintain themselves. Such baffles offer undesirable resistance toflow during such time as afterburning is not in operation. One of themore common forms offixed baflle is a V-shaped gutter having its apexpointing'upstream of the flow. The extreme simplicity of the arrangementtending to outweigh adverse considerations. By setting the arms to anincluded angle of the order of 45, or slightlyless according to flowvelocity, a reasonable optimum between resistance and the provision ofan adequate recirculation zone can be obtained.

Nevertheless baffle resistance can be critical in determining the totalamount of afterburning which can be employed, and an afterburner systemhaving a lower pressure loss when in the nonoperating condition isdesirable, provided that the penalties arising out of otherconsiderations are not too severe. The present invention is concernedwith means directed toward this end, as is British prior copendingApplication. No. 36855/67, now British Pat. No. l,l84,379 and thecorresponding U.S. Pat. No. 3,504,49! to LB. Wigg.

A combustion device according to the invention for installation in gasflow ducts and the like includes at least one member arranged to distortwith the change in temperature and supported in such manner thatincrease in temperature within a predetermined range will distort themember so as to define at least part of a stable combustion zone in aduct.

Preferably, the member is a composite of materials having differentcoefficients of expansion.

An-embodiment of the invention will now be described by way of examplewith reference to the accompanying diagrammatic drawing which representsa longitudinal section of a flame stabilizer.

The stabilizer has a nose portion 1 which comprises a generallyV-section channel member with the extremities 2 of its side webs crankedinwardly to lie parallel with each other. Bimetallic plates 3,4 areriveted to the extremities 2 and extend parallel to each other in acantilever fashion as extensions of the said extremities (i.e. they aredirected away from the apex of the nose portion). The plates 3,4 eachcomprise one sheet of a metal having a relatively high coefficient oflinear expansion, and one sheet of a refractory metal having asignificantly lower coefficient ofexpansion, the two sheets beingattached to each other on mating faces by suitable known methods. Anickel based alloy is a suitable metal for the firstmentioned sheets andniobium, or molybdenum, for the refractory sheets. (A typical nickelalloy has a coefficient of linear expansion of the order of 16 l per C,and that for niobium is about 7.5' per C.) The first-mentioned sheetsface towards each other and the refractory sheets form the outersurfaces of the stabilizer. in some cases it may be expedient toapply anoxidation resistant coating to the sheets.

The stabilizer is intended to be mounted in a gas flow duct with theapex of the nose portion 1 directed towards oncoming flow, the directionof such gas flow being indicated by the arrows B in the drawing.

A tube 5 is located within the nose portion ll lying in the angle of theapex and extending therealong. Formed in the surface of the tube is arow of holes (of which one is shown at 6) extending axially along thetube, the holes being generally directed towards the space formedbetween the plates 3,4. A row of holes (of which one is shown at 7)extends along of the channel member extremities 2 adjacent to the tube5. (There may be differing arrangements of holes in the tube 5 and inthe nose portion 1 --the foregoing being merely typical.)

The tube 5 is connected to a fuel supply (not shown), fuel passing intothe tube being discharged through the holes 6 in its surface, asindicated by the arrow C. Air from the main flow through the duct inwhich the stabilizer is mounted will pass through the holes 7, asindicated by the arrow A to mix with the fuel and the resultant mixture,when ignited by known means, will burn substantially within the confinesof the stabil' izer. The heat of combustion raises the temperature ofthe bimetallic plates 3,4 and the consequent differential expansion ofthe constituent sheets causes the plates to bend from their respectiveattachment points away from each other to take up a configuration asindicated in a dotted line on the drawing. The flame stabilizer as awhole thus becomes in effect a V-shaped gutter having arms set to anincluded angle of approximately 30".

Deflection of the plates will lead to flow over the stabilizer whichhitherto will have been substantially laminar, being induced to breakaway and form a turbulent zone between the plates 3,4. Fuel introducedinto the main flow, as from nozzles 8 located upstream of thestabilizer, and ignited will burn in the said zone where therecirculation existing therein will promote mixing between the hot burntproducts and incoming mixture to maintain stable combustion.

The heat produced as described immediately above will possibly beadequate to maintain the distortion of the bimetallic plates 3,4 and mayeven cause them to deflect further. At this stage it is envisaged thatuse of the pilot burner might be discontinued, though this would bedependent on the operating conditions prevailing and other pertinentfactors.

Likewise, the heat release from the combustion of fuel from the nozzles8 could in certain circumstances cause deflection of the plates 3,4without the need for pilot burning (i.e. the burning of fuel dischargedfrom the tube 5). However, in general, unless ignition of fuel from thenozzles 8 can be carefully controlled, unstable burning conditions wouldbe likely to occur until such time as the plates had deflected toapproximately their intended configuration. In view of the relativelylong time constant possessed by bimetallic constructions generally, thisis most likely to result in the plates not reaching their configuration(or not consistently so). Such uncertainties would, of course, belargely unacceptable and thus the steadystate heating obtainable by theuse of a pilot burner is at least desirable for reliable operation.

In a stabilizer as already described having an overall undeflected widthof one-fourth inch (i.e. as presented to an oncoming flow) with plates0.050 inch thick and 2.8 inches long (in the direction of flow) the freeend of each plate can be deflected outwardly three-eighths inch by atemperature increase of 400c., to give a new overall width of l inch.Thus, the resistance (and resultant pressure loss) when the stabilizeris not in operation is of the order of one-fourth of that of a fixedV-gutter of comparable performance. Deflection of the plates with thematerials envisaged will usually be proportional to the square of theirlength divided by their thickness. Reduction in thickness results inreduced length for a given deflection (with consequent weight reduction)but the effects of gas bending loads, possibility of distortion andstress limita tions must be taken into consideration and also questionsof manufacture.

The stabilizer is envisaged as being installed in, for example,

an afterburner duct between the exhaust of a gas turbine and apropulsion nozzle, in a bypass duct of a turbofan engine, or othersimilar arrangement where, in general, relative shortperiod thrustboosting is required.

In such cases, and particularly in an afterburner, the stabilizer islikely to be subjected to considerable temperatures when it is notitself in operation.

To ensure that the plates extend substantially in the direction of gasflow (asshown in the drawing) 'during such conditions as when anassociated engine is operating but without afterbuming, the plates willbe initially set in the cold condition so that they bend towards eachother and the differential expansion resulting from the temperatureincrease involved immediately above will cause them to straighten out.It may be desirable to fit spacers or other means to prevent the platesfrom bending more than a predetermined amount so as, for instance, toavoid undue stresses.

Also for reasons of stress, the bonding of the sheets forming the platesshould preferably be efiected in a manner which will give no thermalstrain when the plates are undistorted (i.e. at the temperature of thegas stream to which a stabilizer will be subjected in its nonoperatingcondition). Roll bonding or explosion bonding of the sheets can becarried out under nonnal workshop conditions and the plates then rolledat the temperature at which stress relief is desired. Such rolling isnot readily practicable where riveting is used to bond sheets togetherand such a construction is thus less desirable. The effects of stressconcentration and the expansion coefficient of rivets also render thisconstruction generally less satisfactory.

The geometry of a bimetallic stabilizer as heretofore described is notparticularly well suited to a curved configuration (an annulararrangement, for example) but a series of straight stabilizer segmentscan be used to give a close approximation. Alternatively, straightstabilizers could be arranged in parallel, radial or grid fonnations togive a requisite duct coverage.

In the case of an afterburner used with mixed flow from a bypass enginethere is usually a substantial temperature gradient between the core andcircumference of flow through the afterburner duct since the mixingprocess between the hot and cold streams rarely approaches completeness.

In order to use the same bimetallic materials throughout, it might benecessary to vary the stabilizer geometry somewhat in different parts ofthe duct, including, possibly providing stops at the free ends of platessituated in cooler regions to prevent closing of the pilot zones morethan is desirable.

I claim:

1. A combustion device comprising a plurality of members each composedof a plurality of metallic materials bonded together and havingdifferent coefficients of linear expansion, a first metallic materialhaving a predominantly nickel content and a second metallic materialbeing predominantly a refractory metal, said members supported in suchmanner that increase in temperature within a predetermined range willcause them to distort in different directions to define at least part ofa zone wherein stable combustion can be induced to take place.

2. A combustion device according to claim 1 in which the refractorymetal is niobium.

3. A combustion device according to claim 1 in which the refractorymetal is molybdenum.

4. A combustion device according to claim 1 wherein said members aresupported cantilever fashion in such manner that increase in temperaturewill induce the free ends of said members to deflect away from eachother.

5. A combustion device according to claim 1 further comprising integralheating means.

6. A combustion device according to claim 1 wherein said members aresupported on a channel member and define therewith a space, the overallwidth of which will vary with change of temperature.

7. A combustion device according to claim 6 wherein said plurality ofmembers are attached at one end to an extremity of said channel member,fonning an extension of said extremity.

8. A combustion device according to claim 7 further comprising heatingmcans disposed within said space.

9. A combustion device according to claim 8 in which the heating meanscomprises a perforated tube connected to supply fuel to the space.

10. A combustion device according to claim 8 provided with holes in thechannel member for the admission of air to the space.

